(Note to self: V10.0 workbook)
One way is to use dual axis. One axis is the graph. The other is just a mark when/where the event occurs. Change the second axis to SHAPE, and you can mess with whatever shape you want. You can change the colors to contrast with the graph lines if you want. I made the shapes really big to make them stand out.
Once you get the dual axis working right, it's just a matter of window dressing to get the viz the way you want it to look.
See attached. If you need help seeing what I did in there, I'll be glad to provide more detail.
Line Graph Generic A.twbx 31.0 KB
Well that was much easier than I was making it. Thanks!